An energy system is known, for example, from WO 02/14736 A1.
One or more working media such as fuels, propellants or cooling media, is or are required for operation of an energy production device, depending on the nature of the energy produced (for example electrical energy, mechanical energy or thermal energy), the way in which the energy is produced and the operating conditions. One example of this is a fuel cell, whose working media are hydrogen and oxygen.
Particularly for energy systems in mobile applications, there is a requirement for the working media to be stored with a high energy density in as small a space as possible. For this purpose, the working media can be stored in liquid form at high pressure. In submarines, for example, hydrogen and oxygen for fuel cells are thus stored in special pressurized tanks in liquid form at 5-6 bar. However, storage of a working medium in liquid form has the disadvantage that it must be vaporized again, with energy being added, for use in the fuel cell.
Superconductors, and in this case in particular high-temperature superconductors from the family of YBCO conductors or bismuth cuprates, are already being used as low-loss conductors of electrical energy for initial applications in a large number of different electrical devices. These include rotating electrical machines, which operate as motors and/or generators, and whose stators and/or rotors are equipped with windings composed of a superconductor material, or static electrical devices such as current limiters or transformers. One disadvantage of the use of a superconductor is the energy which must be added for a cooling device, with the aid of which the superconductor is cooled to the temperature, normally of 20-100 K, which is required to maintain superconductivity.
The use of fuel cells to produce electrical energy, and of electrical devices with superconductors in a joint system is currently being investigated for widely varying energy systems. These include energy generators, distributors and load installations on marine vessels (see for example WO02/24523A1), large-machine installations and electrical traction systems for heavy-goods transports and locomotives. However, the energy that is required to liquefy the fuel and to cool the superconductor reduces the overall efficiency of energy systems such as these.
WO 02/14736A1 discloses an energy system for a submarine having a fuel cell, and having a current limiter with a high-temperature superconductor. In this case, the superconductor is cooled in a cryostat using liquid nitrogen. The liquid nitrogen is in turn cooled by a first cooling device in the form of a so-called refrigerator. The refrigerator has a cold head which projects into the cryostat, and nitrogen that has been vaporized in the cryostat is liquefied again by recondensation. For this purpose, the cold head absorbs heat from the vaporized nitrogen.
In addition, the electrical power bushings which extend into the cryostat from the outside are cooled with the aid of a second cooling device, in order to reduce the heat that is introduced into the cryostat. The second cooling device has a cooling medium with a boiling point which is higher than the condensation temperature of the liquid oxygen. The second cooling medium is, for example, liquid oxygen for operation of the fuel cell, which is stored in a liquid oxygen tank on board the submarine. The heat transfer from the electrical power supplies to the liquid oxygen results in vaporization of the liquid oxygen and, as a result of its vaporization enthalpy in cooling of the electrical power supplies. This on the one hand reduces the amount of heat introduced into the cryostat, and thus the electrical power consumed by the compressor of the first cooling device, while on the other hand reducing the energy required for vaporization of the liquid oxygen, thus, overall, increasing the overall efficiency of the energy system.